mmc: sdhci-esdhc-imx: Assign system PM ops within #ifdef CONFIG_PM_SLEEP

The system PM callbacks isn't used unless CONFIG_PM_SLEEP is set, thus it
triggers a compiler warning about unused functions. Avoid this by changing
from CONFIG_PM to CONFIG_PM_SLEEP.

Reported-by: Arnd Bergmann <arnd@arndb.de>
Fixes: b70d0b3b5b29 ("mmc: sdhci-esdhc-imx: add esdhc specific suspend resume callback")
Cc: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Dong Aisheng <aisheng.dong@nxp.com>